1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which prints an image on a print medium by ejecting ink onto the print medium and more particularly to a method of controlling voltage pulses applied to electrothermal transducers (heaters) for ejecting ink.
2. Description of the Related Art
The ink jet printing apparatus forms an image by ejecting ink from print elements in response to an image signal to print a plurality of dots on a print medium. Such an ink jet printing system has many advantages over other printing systems, including high speed, high density printing, a color printing capability with a simple construction and a quietness during printing.
A construction that ejects ink from print elements has already been proposed and implemented in some types of printing apparatus, of which a type that uses electrothermal transducers (heaters) in print elements can eject small drops of ink at a high density and at a high frequency and thus has found a wide range of applications. An ink jet print head of this construction has a plurality of print elements arrayed at a density corresponding to a print resolution. Each of the print elements is provided with a liquid path to introduce ink to a nozzle opening and also an electrothermal transducer (heater) in contact with the ink in the liquid path. In ejecting ink from the print elements in response to an image signal, individual heaters are applied a predetermined voltage pulse to be energized to heat the ink. A rapid heating causes the ink in contact with the heater surface to produce a film boiling, in which an expanding bubble expels a predetermined volume of ink from the nozzle opening which flies and lands on a print medium forming a dot.
Further, the ejection volume is influenced by the temperature of the print head or more directly by the temperature of ink near the heater. This is because an ink viscosity changes with an ink temperature and a volume of a bubble and its growth speed during the film boiling depend on the ink viscosity. For example, when the temperature of the print head is low, the ink viscosity increases, making a bubble volume small, with the result that the volume of ink ejected and therefore an area of a printed dot become small. Conversely, when the print head temperature is high, the ink viscosity lowers, making the bubble volume large, with the result that the volume of ink ejected and therefore the printed dot area increase. That is, even if the printing is done based on the same image data, an unstable print head temperature would make the size of dots formed on a print medium unstable, which in turn leads to unstable image density.
Further, when a color image is printed using a plurality of print heads, temperature variations among the different color print heads will likely result in a color produced differing from a desired one. Furthermore, if the temperatures of individual print heads change, the color produced will deviate unstably from target color coordinates.
In the print head manufacturing process, the print heads with a bubble forming heater inevitably have some variations in heater resistance. Considering the print head construction, it is also inevitable that the temperature varies among the print heads depending on the environment in which the printing apparatus is used or the frequencies of use of individual color heads. However, in the ink jet printing apparatus variations in image density and color produced are not desirable. It is therefore one of important tasks with the ink jet printing apparatus to stabilize the ejection volume of the print heads.
Japanese Patent Laid-Open No. 5-031905 (1993) discloses a technology which applies two voltage pulses for each ink ejection and controls a pulse width stepwise according to the temperature of the print head to stabilize the ejection volume of ink. This ejection volume control is referred to as a double pulse drive control. A control circuit of the ink jet printing apparatus sets the pulse width of the pulse signal for ink ejection according to the temperature.
FIG. 19 is a timing chart showing the double pulse drive control. An abscissa represents time and an ordinate represents a voltage applied to the heater. One ejection is done by two pulses shown in the figure. A control circuit in the ink jet printing apparatus sets a pulse width of a pulse signal shown in the figure according to the temperature to stabilize a volume of ejected ink droplets. In the figure, P1 represents a preheat pulse application time, P3 a main heat pulse application time, and P2 an interval between the preheat pulse and the main heat pulse.
The preheat pulse is applied to warm ink near the heater surface and its application time P1 is set so as to keep the energy applied at a level that will not result in generation of a bubble. The main heat pulse on the other hand is applied to cause a film boiling in the ink warmed by the preheat pulse and thereby execute an ejection. Its application time P3 is set larger than P1 so as to produce enough energy to generate a bubble.
As described above, the ink ejection volume is considered as being dependent on a temperature distribution of ink near the heater. Japanese Patent Laid-Open No. 5-031905 (1993) discloses a method which adjusts the pulse width P1 of the preheat pulse according to the detected temperature to realize a stable ejection volume. More specifically, as the detected temperature gradually increases, for example, the necessity of heating the ink near the heater surface decreases progressively. The preheat pulse width P1 is therefore set to decrease progressively. Conversely, when the detected temperature gradually lowers, the necessity of warming the ink near the heater surface progressively increases and the preheat pulse width P1 is set to increase progressively.
The use of this double pulse drive control enables the ejection volume of ink to be kept constant stably for all colors even if the individual print heads have different temperatures at any given time.
In the conventional double pulse drive control such as disclosed in Japanese Patent Laid-Open No. 5-031905 (1993), an energy applied to the heater is adjusted by changing the pulse width while keeping the drive voltage constant. It should be noted, however, that the stabilization of ejection volume can also be achieved with a single pulse by changing the pulse voltage and the pulse width simultaneously. Such ejection volume control methods (hereinafter referred to as single pulse drive controls) are disclosed in Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435.
In the ink jet printing apparatus with a heater, there is a tendency that the ejection volume is larger when a lower voltage pulse is applied for a longer duration than when a higher voltage pulse is applied for a shorter duration. This is considered due to the fact that the application of a lower voltage pulse for a longer duration causes an ink area that is heated up to a bubble forming temperature to spread more widely by heat conduction, whereas applying a high voltage rapidly heats only an area very close to the heater, causing an instant generation of a bubble, resulting in a smaller ejection volume. Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435 describe an ejection control method that takes advantage of such an ejection characteristic and which, when one wishes to increase the ejection volume, reduces the drive voltage and widens (elongates) the pulse width and, when one wishes to reduce the ejection volume, raises the drive voltage and narrows (shortens) the pulse width.
As described above, the ink jet printing apparatus of recent years seek to keep the ejection volume as stable as possible by adopting the double pulse drive control method described in Japanese Patent Laid-Open No. 5-031905 and the single pulse drive control method disclosed in Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435.
The ink temperature in the print head rises as the printing operation continues. So, in the single pulse drive control if one wishes to stabilize the ejection volume in as wide a temperature range as possible, it is preferable to set the drive voltage as low as possible at the start of printing, i.e., at a normal temperature. This is because a lower voltage allows the heat flux to be set lower, reducing the effect a pulse width change has on the ejection volume and thereby making it possible to perform the ejection volume control with precision. It is noted, however, that setting the drive voltage low increases the pulse width required to eject ink, i.e., the time taken by one ejection, resulting in a slower printing speed.
To realize a fast printing, the drive voltage at the lowest temperature at which the print head can print (referred to as a start temperature) needs to be set as high as possible to shorten the time taken by one ejection. This, however, causes the heat flux to be high, rendering the precise control of ejection volume impossible. It also makes the voltage used for the ejection volume control more likely to reach the upper limit of the voltage that the printing apparatus can provide, rendering the ejection volume control itself difficult. Although this problem may be avoided by setting high beforehand the upper limit of the drive voltage that can be supplied to the print head, a circuit that can withstand a higher drive voltage is likely to have an increased circuit area, resulting in an increase in the manufacturing cost. In the ink jet printing apparatus with low cost and small size as one its features, the high voltage drive design is not so practicable.
We have explained the single pulse drive control. The double pulse drive control also has a similar tendency in the relationship between the ejection volume control and the drive voltage. The double pulse drive control keeps the drive voltage at a constant value irrespective of the ink temperature. Depending on whether this constant value is set relatively low or high, the precision of the ejection volume control and the printing speed vary.
The double pulse drive control reduces the preheat pulse width progressively as the temperature rises, to keep the ejection volume within a predetermined range. So, basically the ejection volume control can be performed in a temperature range from the start temperature to a temperature at which the preheat pulse width becomes zero. If the drive voltage is set relatively low, the heat flux from the heater to the ink is small so that a change in the preheat pulse width has little effect on the ejection volume, allowing for a correspondingly more precise ejection volume adjustment. Further, since the preheat pulse width at the start temperature is relatively long, the ejection volume control can be executed in a wide temperature range up to the temperature where the preheat pulse width becomes zero. It should be noted, however, that, as with the single pulse drive control, the longer pulse width results in each ejection taking longer and the printing speed getting slower.
When, on the other hand, the drive voltage is set relatively high, the preheat pulse width at the start temperature can be set short beforehand, allowing for a faster printing speed. However, since the heat flux from the heater to the ink at time of preheat pulse application is high, the effect a change in the preheat pulse width has on the ejection volume increases. This means that the adjustment of the ejection volume becomes that much coarse. Further, since the preheat pulse width at the start temperature is short, even a slight temperature change can result in the preheat pulse width becoming zero, narrowing the temperature range where the ejection volume control can be performed normally.
Recent years have seen the use of the ink jet printing apparatus grow in versatility and there are growing needs for a capability to print a variety of kinds of images on various types of print mediums. One such example is a demand for printing on a sheet of glossy photographic paper a picture image with as stable a color as will match that of a silver salt picture. To meet this demand requires an ejection volume control with high precision and reliability. At the same time, there is also a call for printing monochrome text images on low-cost plain paper at high speed. To meet this demand requires reducing a drive pulse width. Under these circumstances, the conventional ink jet printing apparatus have difficulty meeting the two needs of the users—the image quality and the printing speed—at the same time.